Two recent technological hallmarks have been the development of the personal computer and the wireless mobile telephone or cellular phone. In fact, the last ten years of the twentieth century have been marked by unprecedented growth in the demand for personal computers, particularly laptops, and wireless telephones (or cell phones). The personal computer owes its popularity mainly in part to its ability to access and process relatively large amounts of data, its price, and its size, especially in the case of laptops. Specifically, a personal computer allows for accessing and processing large amounts of multimedia information available, for example, via the Internet from the top of a desk or the lap of a user. Consumers via Internet access can send and receive email messages, preview movies, research intended purchases, etc. In essence the Internet and personal computer have made the consumer smarter through access to a heretofore unimaginable plethora of information.
Cell phones, on the other hand, have allowed users mobility previously unavailable by wireline phones. Specifically, whereas a wireline phone restricts the user's mobility to the location of the phone, a user may make and receive calls from a cell phone even while roaming over a very large geographical area such as the contiguous United States. In addition, as the user roams geographically the quality of service is maintained at a fairly high level.
Merging the mobility of the cellular network with the information capability and accessibility of the Internet has become a main focus of the communications industry. In particular, in recent years considerable research has been directed to developing mobile protocols that would allow seamless access to the multimedia services available on the Internet thereby allowing consumers access anytime and anywhere.
The Internet is a packet data network in which the Internet Protocol (IP) defines the manner in which a user is connected to the Internet so as to access, transmit, and receive information from other users or resources connected to the Internet. In particular, in accordance with IP each network access point is identified by an IP address. When a user attaches to a particular network access point the user (more precisely, its terminal) is given an IP address. The addresses available at access point are assigned geographically. Consequently, as a user roams geographically the user's point of attachment to the network changes which in turn requires the user's IP address to change. Further, information destined for a user, or resource, is packetized with each packet having the IP address of the user, more accurately the user's terminal, in a header. As packets traverse the network, the IP address included in the header is used to route the packet to its destination. Thus, as a user roams and her IP address changes, the route to the user changes which in turn may affect the quality of service for some multimedia services, i.e., real time services, as there is no guarantee that network resources required to support the service are available. At a fundamental level IP was not designed with mobility in mind as evidenced by the manner in which IP addresses are assigned.
In contrast, the wireless telephone network is a circuit switched network with each user's telephone number serving as a unique access identifier. Consequently, as the user roams geographically the user's identity is unchanged thereby allowing the network to easily track the user's movement, establish new circuits in anticipation of the user moving to a different geographic region, and maintain the needed quality of service. In addition, in the wireless telephone network calls between users are routed through the network on circuits that are established for the duration of the call. In other words, a path is established in the network for exclusively carrying each call thereby assuring the user of the bandwidth needed for the service.
Given the fundamentally different approaches underlying the manner in which access is provided by the Internet and by the wireless telephone network and the manner in which paths are established and signals routed through each of these networks, many issues need to be resolved before multimedia services can be provided over an IP wireless Code Division Multiple Access (CDMA) network. Nonetheless, forecasts indicate that users or consumers will ultimately desire accessing currently available and future multimedia services available via the Internet while being mobile, i.e., combining the cell phone mobility with the processing power of the personal computer. As such, there has been an international effort to provide mobile access to Internet protocols.
Responding to this apparent demand, the International Telecommunications Union (ITU) promulgated International Mobile Telecommunications—2000 (IMT-2000) global standards to allow for wireless access to multimedia information or services available via the Internet in much the same way consumers are use to using their cell phones, so called third generation wireless (3G wireless) services. The IMT-2000 standards have made significant progress in defining a common radio system architecture, including services, interfaces, and radio spectra. For example, at the physical layer, IMT-2000 includes standards on the frequency of the chip sets used to support the services and the radio frequency spectrum, which will be used for the services. By physical layer we refer to the first layers of the 7-layer Open System Interconnect (OSI) reference model wherein the layers are ordered as follows: layer 1 is the physical layer and the lowest layer in the stack, layer 2 is the link layer and above layer 1, layer 3 is the network layer and above layer 2, layer 4 is the transport layer and above layer 3, layer 5 is the session layer and above layer 4, layer 6 is the presentation layer and above layer 5, and layer 7 is the applications layer and the highest layer. IMT-2000 includes definitions on upper layer protocols, but mostly for circuit based networks. IMT-2000 also includes standards on Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) technologies.
There are two basic CDMA standards that have been identified under the IMT-2000 standard, Code Division Multiple Access (CDMA) 2000 and Wideband-CDMA (W-CDMA), for third generation wireless networks. The ITM-2000 standards in turn have spawned numerous industry organizations and groups all with the general goal of developing applicable technical specifications for supporting CDMA 2000, W-CDMA, and third generation TDMA systems. Some of these organizations include the 3rd Generation Partnership Project (3GPP), the 3rd Generation Partnership Project 2 (3GPP2) and the Mobile Wireless Internet Forum (MWIF). These organizations are directing their efforts to solving the problems that will be encountered in trying to provide 3G wireless multimedia services or mobile access to Internet services.
Of particular import to the present invention is the function or feature within a wireless network called soft handoff. In a conventional prior art wireless network such as shown in FIG. 1A, a plurality of base stations 10 of which base stations 101 and 102, are depicted transmit or send information over the air to a plurality of mobile units 20. The range within which a mobile unit 20 (can reliably receive information from a base station 10 defines a cell 21. As illustrated in FIG. 1A the cells 211, 212, 23 - - - 21k may be depicted as a honeycomb structure. As a mobile unit 202 for example, roams and moves further away from a base station 102 corresponding to cell 212 for base station 102, signal strength decreases. Further, as the mobile moves from one cell to another, the mobile station needs to switch from the serving base station, the base station for the cell it currently is in, to a target base station, the base station for the cell that it is moving to. The process of the mobile switching base stations is known as handoff.
Handoff can be hard or soft. In a hard handoff a user may receive data from only one base station at any given time. In other words, there is a single wireless data transport path for a user at any given time and the path has to change when the user moves from one cell to another. This could cause data in transit, e.g., data that has been sent to the previous serving base station, to be lost during hard handoff therefore causing performance degradation.
In a soft handoff, the user seamlessly switches from one base station to the next without any perceptible degradation in service. During a soft handoff a mobile user communicates with multiple base stations simultaneously. Therefore, a user may be able to switch to a new base station without data loss. Soft handoff is the method of choice employed in the conventional CDMA wireless network. In addition, soft handoff must be supported in 3G wireless networks, as it would be awfully inconvenient for a user's service, e.g., a video conference, to be disrupted each time the user switches base stations.
In addition to providing for seamless service, soft handoff also allows cells to cover a larger geographic area. This is the case because during soft handoff the mobile unit receives signals from at least two base stations and combines these received signals to obtain the information intended for the user. Because it receives two or more signals, each signal can be at a lower level than if the mobile were receiving only one signal. Accordingly, each base can be allowed to cover a larger geographic area.
The network of FIG. 1B is currently able to support cellular telephony and limited data transmissions, e.g., 9.6 kb/s for GSM and 14.4 kb/s for CDMA, and is usually referred to as 2G wireless network. With reference to FIG. 1B we will illustrate how soft handoff occurs in today's network. A user's mobile unit 20 is communicating with its serving base station 105 in the corresponding cell 215. The base station 105, and probably mobile 20, monitor the signal strength of the mobile unit 20 and when the mobile's signal strength drops below a pre-specified level soft handoff is initiated. That is, as the mobile enters the soft handoff region 33, the base station 105 and the mobile unit 20 together initiate the appropriate steps through a base station controller 35 and a mobile switching center 40, if necessary, in circuit switched network 47 to locate the target base station 106 for the neighboring cell 216 serving the same soft handoff region 33. Note that the mobile switching center would not be included in soft handoff, given the current illustrative example, because both base stations are controlled by the same base station controller. Identical information intended for the mobile unit 20 is then routed to both the target base station 105 and the serving base station 106. Both base stations in turn transmit the identical information to mobile unit 20. The mobile unit 20 then combines the signal to produce the information intended for the user. As the mobile unit 20 leaves the soft handoff region 33 and enters the target cell 216, soft handoff is terminated and the target base station 106 becomes the only base station serving the mobile unit 20. In a similar manner the mobile unit is handed from base station to base station as the unit roams from cell to cell.
It is important to note that the backbone of the network of FIG. 1B is entirely circuit switched. It is also important to note that in order for the mobile unit to combine the signals that are received during soft handoff, the signals must be identical at the physical layer and synchronized. In FIG. 1C we further illustrate the impact of requiring that identical data be present at the physical layer during soft handoff in today's centralized network. As FIG. 1C shows, in the forward direction or downlink (from base station to mobile station), a centralized Selection and Distribution Unit (SDU) 60, typically residing in a mobile switching center, is responsible for distributing traffic, over layer-2 circuits, via different base stations to the mobile station and ensuring that the matching link-layer (and physical-layer) frames sent to different base stations, 105 and 106, contain copies of the same data, i.e., data content synchronization. In the reserve direction, the mobile station ensures that the matching link-layer frames sent to different base stations contain copies of the same data.
The mobile station must also collaborate with the base stations to synchronize the radio channel frames received (and transmitted) by the mobile station, i.e., frame synchronization.
In the forward direction, the mobile station combines the radio signals received in the matching frames from different base stations to generate a single final copy of each piece of received data. In the reverse direction, the SDU combines the matching data received from different base stations, data content combination.
Ultimately the network architecture of FIG. 1B will transition to the IP-based autonomous wireless base stations network of FIG. 1D. In comparing the architecture of FIG. 1D to FIG. 1B, we note the following important differentiating features of FIG. 1D: (1) base stations 100 function autonomously, i.e., there are no base station controllers or mobile switching centers to centrally control the base stations; (2) the backbone network 107, including connections 117 that interconnect the base stations 100, is an all IP network, as opposed to a circuit switched network; and (3) the base stations are capable of performing IP layer processing, e.g., forwarding packets based on information in the IP headers, signaling, and mobility management. Because the base stations 100 function autonomously and are interconnected via an all IP backbone network 107 prior art methods will not support soft handoff in the network of FIG. 1D.
There is a need therefore for methods, systems, and circuitry for supporting soft handoff in an IP-based autonomous wireless base station network. Specifically, such methods, systems, and circuitry must meet the following requirements for supporting soft handoff:                1. Separate copies of the same data need to be sent by different base stations to the same mobile simultaneously and in the reverse direction the mobile unit needs to ensure that the same data is sent to the base stations, i.e., data content synchronization,        2. A mobile station's radio system has to be able to recognize which of the radio frames carrying data from different base stations contain copies of the same data so that it may synchronize the frames for combining the data content, i.e., frame synchronization, and        3. A mobile station should be able to combine copies of the same IP packet into a single packet and present the combined packet to the applications, i.e., data content combination.        